1. Field of Invention
The present invention relates to a heat dissipation device. More particularly, the present invention relates to a circulative cooling apparatus.
2. Description of Related Art
Electronic devices are developed to have smaller volume, lighter weight, and lower power consumption as science and technology makes tremendous progress. Energy utility rates of electronic devices are not completely utilized since a lot of energy is consumed as heat energy. The heat energy arises a temperature inside an electronic system. When the temperature inside the electronic system is higher than that can be tolerated by the electronic devices thereof, the physical characteristics of the electronic devices are changed by the high surrounding temperature and the electronic system becomes abnormal, generates errors, or is halted. Furthermore, the temperature inside the electronic system is raised and the breakdown rate of the electronic system is higher.
A main object of heat dissipation is to enhance reliability of a system. A temperature surrounding the electronic devices inside the system has a great effect on the reliability of the system. The temperature inside the system must be maintained at a lower degree to obtain a higher reliability of the system. Heat dissipation devices are therefore used to lower the temperature inside the system to an ideal operating temperature range for the electronic devices inside the system. Heat dissipation device design concerns an operating environment of the system, operating conditions, and allowed operating temperatures of the system. A structure of the system with good heat dissipation shapes or apparatus can dissipate heat outside the system and maintain a constant temperature therein to keep the system stable.
In conventional systems, heat dissipation of electronic devices usually uses fans driven by electric power to dissipate heat absorbed by heat sinks, so as to force airflows surrounding electronic devices to convect. But disadvantages of this method are large power consumption, noise and vibration that affects operations of electronic devices. Therefore, various apparatus have been developed to replace conventional fans/heat sinks combinations, such as a heat dissipation apparatus using phase changes of materials.
The heat transferring efficiency of two phase flow heat removal is higher than the heat transferring efficiency of single phase flow heat removal. A feature of the two phase flow heat removal is use of the latent heat between two phases of a work fluid to remove rapidly a great amount of heat energy. Some conventional apparatus, such as heat pipes and vapor chambers, utilizes this method. The following description explains them respectively.
FIG. 1 is a schematic view of a structure of a conventional heat pipe 100 and operating processes thereof. As shown in FIG. 1, the heat pipe 100 is a hollow container with low pressure therein. Materials, such as sintered metal, or feltmetal, are used to make a porous structure 114 on internal walls of a pipe case 112. The porous structure 114 is soaked in a work fluid. The work fluid can be water, mercury, Freon, sodium, potassium or silver, depending upon what working temperature is needed.
A side of the heat pipe 100 is an evaporation section 102. The evaporation section 102 contacts heat energy 126 of a heat source, and the work fluid within the evaporation section 102 is evaporated to become vapor because of absorbing heat energy 126 from the heat source. Owing to the pressure drop the vapor forms a vapor flow flowing to a condensation section 104, and then condenses into the original work fluid. The condensed work fluid falls to the porous structure 114 inside the condensation section 104, and afterward forms fluid flow 124 to flow back to evaporation section 102 due to capillary action. Evaporation and condensation are repeated to dissipate heat.
FIG. 2 is a schematic view of a structure of a conventional vapor chamber 200 and operating processes thereof. As shown in FIG. 2, the vapor chamber 200 is a hollow container with a low pressure therein. Internal walls of the vapor chamber 200 are like the heat pipe 100 as illustrated in FIG. 1, having a porous structure 214, and the porous structure 214 is also soaked in a work fluid. A side of the vapor chamber 200 is an evaporation section 202.
The evaporation section 202 contacts heat energy 226 of a heat source, and the work fluid within the evaporation section 202 is evaporated to become vapor by absorbing heat energy 226 from the heat source. The vapor moves upward and contacts an upper chamber wall 212 on the top of the evaporation section 202, and then dissipates heat therein and condenses back to the original work fluid. The condensed work fluid falls back to the evaporation section 202 due to gravity or other reasons. Evaporation and condensation are repeated to dissipate heat.
The method of two phase flow heat removal continuously removes heat energy from a heat source by a successive circulation of a work fluid. The work fluid absorbs and dissipates latent heat of phase change to transfer heat energy, and the thermal conductivity thereof is a hundred or thousands of times greater than that of the thermal conductivities of silver or copper. For example, a effective thermal conductivity of a simplest sodium heat pipe (using stainless steel as it's the pipe case, rolling stainless steel net as it's the porous structure, and sodium as it's the work fluid) is over 418 J/(cm·s·° C.), but a thermal conductivity of copper is only 3.8 J/(cm·s·° C.).
However, since the work fluid and the vapor thereof both exist in the same space, when the evaporation rate of the vapor and the condensation rate of the work fluid reach critical values, entrainment and counter current flow are generated. These increase the instability between the work fluid and the vapor, and further decrease the condensed work fluid that should flow back to the evaporation section for a next evaporation procedure. If the work fluid cannot flow back to the evaporation section successfully, the heat energy of the evaporation section does not dissipate by the circulative work fluid. Then the temperature of the evaporation is continuously raised until the pipe is dried out and disabled.